The manufacture of semiconductors results in the generation of an exhaust gas containing poisonous gases or gases which may cause environmental pollution such as SiH.sub.4, Si.sub.2 H.sub.6, PH.sub.3, NH.sub.3, NF.sub.3 SF.sub.6, CF.sub.4 and C.sub.2 F.sub.6. It is therefore necessary in facilities that discharge such exhaust gases to install an exhaust gas treatment unit to thereby decompose the exhaust gas until the concentration of these poisonous gases reaches an allowable concentration (TLV value) specified by law.
Known types of exhaust gas treatment units involve the use of a combustion type unit in which the exhaust gas is burnt by flame to effect decomposition. A general combustion type exhaust gas treatment unit utilizes a flame from a burner that is blown to the exhaust gas so that the exhaust gas is decomposed by its heat. Such a combustion type exhaust gas treatment unit can work effectively in the context of combustible gases such as SiH.sub.4, Si.sub.2 H.sub.6, PH.sub.3 and NH.sub.3.
However, in this known type of combustion type exhaust gas treatment unit, the flame is rather short from a duration standpoint. On the other hand, gases such as NF.sub.3, SF.sub.6, CF.sub.4 and C.sub.2 F.sub.6 tend to be very stable gases. Thus, a large-sized unit having a large capacity must be used and a large amount of fuel is required to decompose these gases.
In light of the foregoing drawbacks and limitations associated with known types of units, the present invention has been developed.
It is an object of the present invention to provide an exhaust gas treatment unit of the combustion type that is capable of carrying out the decomposing treatment of an exhaust gas effectively, while at the same time permitting the unit to be down-sized.
An exhaust gas treatment unit according to the present invention includes a treatment drum, a combustion air introduction tube attached at one end of the treatment drum to introduce combustion air to the inside of the treatment drum, a fuel gas introduction tube disposed inside the combustion air introduction tube to introduce fuel gas to the inside of the treatment drum, an exhaust gas introduction tube disposed inside the fuel gas introduction tube to introduce exhaust gas to the inside of the treatment drum, and a flame tube extending from the end portion of the combustion air introduction tube on the air outlet side in the vicinity of the other end of the treatment drum. A cooling air introduction device is provided to introduce cooling air into a space between the side wall of the treatment drum and the flame tube, and an outlet nozzle is provided at the other end of the treatment drum to discharge the treated exhaust gas.
A process to dispose of exhaust gas from a semiconductor manufacturing unit in accordance with the present invention involves injecting exhaust gas into a drum, reacting the exhaust gas with a flame of a fuel gas and oxygen containing gas in the drum in an oxidizing reaction to produce oxidized products, and controlling the temperature of the oxidizing reaction by cooling the wall of the drum. The oxidized products are filtered to remove solid particles and produce gaseous species which are then scrubbed to release only non-harmful species to the environment.
If a mixed gas of fuel gas and combustion air is ignited in such construction as mentioned above, a flame will go ahead through the flame tube, whereby a high heat situation can be maintained. As a result, the contact of the flame with the exhaust gas is effectively carried out.
To permit treatment of plural kinds of exhaust gases at the same time, a plurality of exhaust gas introduction tubes may be provided. From a similar point of view, the inside of one exhaust gas introduction tube may be divided into a plurality of exhaust gas introduction paths.
Further, a powder scrape-off means may preferably be provided in the vicinity of the end portion of the exhaust gas introduction tube on the exhaust gas outlet side so that oxides generated in the treatment of the exhaust gas such as SiH.sub.4 are not deposited on the fore end of the exhaust gas introduction tube.
Oxygen enriched gas, e.g., air (containing approximately 21% oxygen) or oxygen enriched air, including substantially pure oxygen, can be used for the combustion, particularly to reach high temperatures necessary to decompose certain species.
The exhaust gas treatment unit of the combustion type according to the present invention can maintain a flame for a long period of time and can carry out its contact with an exhaust gas effectively because of the flame tube that is provided. Thus, it becomes possible to achieve the down-sizing of the unit, and even in a case where a small amount of fuel is used, it is possible to effectively carry out the decomposing treatment of stable exhaust gas, for example an inorganic fluorine compound such as NF.sub.3 of SF.sub.6, or any perfluorocompounds (PFCS) such as CF.sub.4, C.sub.2 F.sub.6 or the like.
Since cooling air flows around the flame tube, there is no need to provide a heat insulator on the outer circumference of the exhaust gas treatment unit. This further facilitates the ability to downsize the unit.
By mixing the cooling air with the treated exhaust gas in the exhaust gas treatment unit, the diluting treatment of exhaust gas is carried out during cooling and so the exhaust gas treatment becomes more effective.